SECTION 3.3
87
Filters
Viewing and printing applications may gain performance benefits by using the
resolution progression. If the full-resolution image is densely sampled, the
application may be able to select and decode only the data making up a lower-
resolution version, thereby spending less time decoding. Fewer bytes need be
processed, a particular benefit when viewing files over the Web. The tiling
structure of the image may also provide benefits if only certain areas of an image
need to be displayed or printed.
Note:
Information on these progressions is encoded in the data; no decode parame-
ters are needed to describe them. The decoder deals with any progressions it encoun-
ters to deliver the correct image data. Progressions that are of no interest may simply
have performance consequences.
The JPEG2000 specifications define two widely used formats, JP2 and JPX, for
packaging the compressed image data. JP2 is a subset of JPX. These packagings
contain all the information needed to properly interpret the image data,
including the color space, bits per component, and image dimensions. In other
words, they are complete descriptions of images (as opposed to image data that
require outside parameters for correct interpretation). The
JPXDecode
filter
expects to read a full JPX file structure—either internal to the PDF file or as an
external file.
To promote interoperability, the specifications define a subset of JPX called
JPX
baseline
(of which JP2 is also a subset). The complete details of the baseline set of
JPX features are contained in ISO/IEC 15444-2,
Information Technology—JPEG
2000 Image Coding System: Extensions
(see the Bibliography). See also
Data used in PDF image XObjects should be limited to the JPX baseline set of
features, except for enumerated color space 19 (CIEJab). In addition, enumerated
color space 12 (CMYK), which is part of JPX but not JPX baseline, is supported in
PDF.
A JPX file describes a collection of
channels
that are present in the image data. A
channel may have one of three types:
•
An
ordinary
channel contains values that, when decoded, become samples for a
specified color component.
•
An
opacity
channel provides samples that are to be interpreted as raw opacity
information.